Hierarchical porous metals with deterministic 3d morphology and shape via de-alloying of 3d printed alloys

ABSTRACT

The present disclosure relates to a system for using a feedstock to form a three dimensional, hierarchical, porous metal structure with deterministically controlled 3D multiscale porous architectures. The system may have a reservoir for holding the feedstock, the feedstock including a rheologically tuned alloy ink. A printing stage may be used for receiving the feedstock. A processor may be incorporated which has a memory, and which is configured to help carry out an additive manufacturing printing process to produce a three dimensional (3D) structure using the feedstock in a layer-by-layer fashion, on the printing stage. A nozzle may be included for applying the feedstock therethrough onto the printing stage. A de-alloying subsystem may be used for further processing the 3D structure through a de-alloying operation to form a de-alloyed 3D structure having several distinct, differing pore length scales ranging from a digitally controlled macroporous architecture to a nanoporosity introduced by the de-alloying operation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional and claims the benefit and priority ofU.S. patent application Ser. No. 15/790,810 filed on Oct. 23, 2017. Theentire disclosure of the above application is incorporated herein byreference.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Contract No.DE-AC52-07NA27344 awarded by the United States Department of Energy. TheGovernment has certain rights in the invention.

FIELD

The present disclosure relates to the formation of porous metals, andmore particularly to the formation of 3D periodic porous materialshaving an engineered, hierarchical, multi-porosity structure.

BACKGROUND

The statements in this section merely provide background informationrelated to the present disclosure and may not constitute prior art.

Conventionally, nanoporous metals with uniform single level porosityhave been fabricated by de-alloying methods from an alloy precursor. Theperformance of these materials in many applications often suffers frommass transport limitations, which is specifically true for monolithicmacroscopic nanomaterials. In the extreme case, mass transportlimitations limit reactions to the geometrical surface of themacroscopic monolithic material, thus leaving the majority of theinternal surface within the nanoporous bulk material unused (see, forexample FIG. 1). This limitation can be overcome by introducing ahierarchical structure with at least two levels of pore sizes: 1) largepores which act as mass transport “highways” that allow the reactants todiffuse to small pores, while 2) nanosized pores provide high surfacearea and thus are responsible for functionality.

Hierarchical nanoporous gold has been realized using template methods,for example by injecting a target metal salt to replicate the structureof a hierarchical template (Lee et al. “Developing Monolithic NanoporousGold with Hierarchical Bicontinuity Using Colloidal Bijels” J Phys Chem.Lett., 2014, 5, 809). However, pore size distributions and sampledimensions are determined by the template which is difficult to tune atdifferent levels of structures. Specifically, anisotropic templates aredifficult to realize by nature's self-organization and self-assemblymethods. For example, Lee and his co-workers (Lee et al., supra) usedcolloidal Bijels as template materials. By filling the template withHAuC14 and AgNO3 solutions with the desired composition, and thenfollowed up with an annealing process to form the alloys and remove thetemplate. Finally, a de-alloying process was used to remove Ag from AgAualloys.

Bulk hierarchical nanoporous gold has also been prepared using amulti-step corrosion-deposition-annealing-de-alloying process (see Qi etal. “Hierarchical Nested-Network Nanostructure by De-alloying” ACS Nano,2013, 7, 5948). The corrosion-deposition-annealing-de-alloying processis limited by the availability of suitable starting alloys. Solidsolution alloys which present only a tiny fraction of binary alloys areso far the only reported system for this process. For example, Qi andWeissmueller (Qi et al., supra) chose a dilute AgAu alloy with the goldcontent of 5 atomic percent as a starting alloy, whereas the normal goldcomposition range for dealloying is 20-50 atomic percent. They then usedan electrochemically controlled de-alloying process to partially removeAg from the dilute AgAu alloy to form a nanoporous AgAu alloy with ahigh residual Ag content, which is enough to perform a secondde-alloying process. It should be noted that the normal compositionrange for dealloying cannot achieve such a high residual Agconcentration, and therefore it is not possible to perform a secondde-alloying process. The obtained nanoporous AgAu alloy was annealed at30020 C. for 3 hours to form the upper hierarchy structure with aligament size of ˜200 nm. A second de-alloying process introduces thelower level hierarchical structures with a size of ˜20 nm. However, thisprocess also does not allow for the realization of anisotropic porearchitectures required for directed mass transport.

SUMMARY

In one aspect the present disclosure relates to a system for using afeedstock to form a three dimensional, hierarchical, porous metalstructure with deterministically controlled 3D multiscale porousarchitectures. The system may comprise a reservoir for holding thefeedstock, the feedstock including a rheologically tuned alloy ink. Aprinting stage may be included for receiving the feedstock. A processorincluding a memory may be included which is configured to help carry outan additive manufacturing printing process to produce a threedimensional (3D) structure using the feedstock in a layer-by-layerfashion, on the printing stage. A nozzle may be included for applyingthe feedstock therethrough onto the printing stage. A de-alloyingsubsystem may be included for further processing the 3D structurethrough a de-alloying operation to form a de-alloyed 3D structure havingseveral distinct, differing pore length scales ranging from a digitallycontrolled macroporous architecture to a nanoporosity introduced by thede-alloying operation.

In another aspect the present disclosure relates to a system for forminga three dimensional, hierarchical, porous metal structure withdeterministically controlled 3D multiscale hierarchical porearchitectures. The system may comprise a printing stage and an additivemanufacturing system having a processor and a nozzle. The additivemanufacturing system may be configured to print a three dimensional (3D)structure in a layer-by-layer process by flowing a rheologically tunedink through the nozzle onto the printing stage, and to build up the 3Dstructure in a layer-by-layer operation. The system may also include anannealing subsystem configured to anneal the 3D structure to remove thebinder, and to form an alloyed 3D structure. The system may also includea de-alloying subsystem configured to de-alloy the alloyed 3D structureto form a hierarchical, nanoporous 3D structure having an engineered,digitally controlled macropore morphology with integrated nanoporosity.

In still another aspect the present disclosure relates to a system forforming a three dimensional, hierarchical, porous metal structure withdeterministically controlled 3D multiscale hierarchical porearchitectures. The system may comprise a printing stage, a rheologicallytuned, flowable ink including a metal powder and a binder, and anadditive manufacturing system having a processor for controlling aprinting process. The additive manufacturing system may have a nozzleand may be configured to print a three dimensional (3D) structure in alayer-by-layer process by flowing the rheologically tuned ink throughthe nozzle onto the printing stage. In this manner the 3D structure isbuilt up in a layer-by-layer printing operation. An annealing subsystemmay be configured to anneal the 3D structure by heating the 3D structurefor a predetermined time period to remove the binder, to form an alloyed3D structure. A de-alloying subsystem may be included which isconfigured to de-alloy the alloyed 3D structure to form a hierarchical,nanoporous 3D structure. The hierarchical, nanoporous 3D structure hasan engineered, digitally controlled macropore morphology with integratednanoporosity.

Further areas of applicability will become apparent from the descriptionprovided herein. It should be understood that the description andspecific examples are intended for purposes of illustration only and arenot intended to limit the scope of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings described herein are for illustration purposes only and arenot intended to limit the scope of the present disclosure in any way.

FIG. 1 is a diagram illustrating the mass transport limitations of priorart non-hierarchical nanoporous materials formed through conventionalde-alloying methods;

FIG. 2 shows a simplified diagram illustrating how various metalparticle clays, together with a solvent, may be mixed together to forman ink that may be used in a direct ink writing (DIW) process;

FIG. 3 is a simplified diagram of using the ink shown in FIG. 2 in a DIWoperation to form a 3D structure in a layer-by-layer process;

FIG. 4 shows a 3D printed Au—Ag alloy structure after an annealingoperation has been performed to burn off the binder material and toalloy the metal components of the ink;

FIG. 5 shows a 3D printed, hierarchical porous structure created througha de-alloying process, which leaves a 3D structure having a plurality ofdistinct porosity length scales

FIG. 6 shows a cross sectional view of a portion of the hierarchical 3Dstructure shown in FIG. 5 to illustrate the increase in mass transportand accessible contact area of the material for charge carriers, ascompared to the prior art 3D structure of FIG. 1;

FIG. 7 shows Scanning Electron Micrographs at different magnificationsof an example 3D structure created using the teachings of the presentdisclosure, in which the 3D structure has three distinct porosity lengthscales;

FIGS. 8-11 illustrate examples of complexly shaped 3D structures thatmay be created using the teachings of the present disclosure;

FIG. 12 shows a cyclic voltammetry graph illustrating theelectrochemical surface area of a hierarchical 3D structure createdusing the teachings of the present invention compared to aconventionally created unimodal 3D structure with the same thickness;

FIG. 13 shows a graph comparing the electrical current response ofhierarchical (solid lines) and non-hierarchical (dash lines) nanoporousgold electrodes in response to a sudden change of the appliedelectrochemical potential from E_(i)=0 V to E_(F) wherein the differentcolors from top to bottom represent different values of E_(F) rangingfrom 0.1V to 0.6V; it can be seen that the hierarchical structurecharges always faster compared to the unimodal nanoporous gold; and

FIG. 14 is a high level flowchart summarizing basic operations that maybe performed in forming hierarchical nanoporous metal foams and other 3Dstructures.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is notintended to limit the present disclosure, application, or uses. Itshould be understood that throughout the drawings, correspondingreference numerals indicate like or corresponding parts and features.

The present invention uses an additive manufacturing operation, in oneexample a DIW additive manufacturing process, to fabricate hierarchicalnanoporous metal foams with deterministically controlled, applicationspecific, 3D multiscale pore architectures. Arbitrary macroscopicarchitectures and sample shapes can be printed according to theapplication requirements. Moreover, the structure of two, three, or moredistinct levels of porosity can be tuned independently which enablesapplication specific multiscale architectures of virtually any geometric3D shape.

Referring to FIGS. 2-5, an additive manufacturing process, in thisexample a direct ink writing (DIW) additive manufacturing method, may beused to deposit filaments of rheologically tuned alloy “inks” made fromdesired metal powder mixtures, in a predefined geometry, for example anet-shaped, porous form. At FIG. 2, a metal particle mixing operation isperformed. In this example gold clay 12 and silver clay 14 are mixedtogether as powders with a solvent(s) acting as an organic binder 16.However, it will be appreciated that other combinations of metal powdersmay be selected, and the present disclosure is not limited to use withonly Au and Ag powders. In this example, however, the specificquantities and/or ratio of Au and Ag (e.g., metal powders) may beselected, along with the quantity of organic binder(s) 16, and mixedtogether as colloids/particles to tune the rheological properties of thecomposition. The composition forms an ink 18 after being mixed. Thus,the operations performed in FIG. 2 may be thought of as a “mixing” orpowdered metal ink preparation process. Changes to the metal powdermixing ratio (e.g., Au powder from Au clay 12 and Ag powder from Ag clay14) allow adjustment of the metal alloy composition formed during asubsequent DIW/annealing process. Alternatively, premade alloy particleswith the desired metal component composition can be mixed with thebinder (i.e., solvent) 16 to prepare the alloy ink.

In FIG. 3, an ink reservoir 20 may be used to hold the ink 18 and supplythe ink to a nozzle 20 a of a DIW system 22. The present disclosure isnot limited to any particular construction of DIW system, but in thisexample the DIW system 22 includes a computer controlled processor 24and a memory 26. The DIW system uses the nozzle 20 a to form a 3Dprinted, structure 28 in a “layer-by-layer” fashion. The 3D structure 28formed initially by the DIW system 22 may be termed a “green part” toindicate that further manufacturing operations are to be performed onthe parts. Illustration 28′ in FIG. 3 illustrates a plan view showingone example of the porosity of the 3D structure 28 formed by printedmetal particle ink filaments that form a lattice-like structure.

In this example the DIW operation using the Ag—Au alloy forming metalparticle mixture (i.e., ink 18) forms an extrusion-based, roomtemperature manufacturing process. The Ag—Au ink 18 in this example washoused in a 3 cm×3 cm syringe barrel (EFD) (shown as nozzle 20 a)attached by a Luer-Lok to a smooth-flow tapered nozzle (200 micronsinner diameter, “d”). An air-powered electronically controlled fluiddispenser, in this example the ULTIMUS™ V, EFD (available from theNordson Corp. of Westlake, Ohio), provided the appropriate pressure toextrude the ink 18 through the nozzle 20 a. The extrusion process may becontrolled by controlling the extrusion pressure and printing speedduring the writing operation. The target patterns forming the 3D Au—Agparticle structure 28 in this example were printed using an x-y-z 3-axisair bearing positioning stage (model ABL 9000, available from Aerotech,Inc. of Pittsburgh, Pa.), whose motion was controlled by writing theappropriate G-code commands. The 3D Ag—Au metal particle structure 28was printed in a layer-by-layer scheme onto silicon wafers with a nozzleheight (h) of 0.7 d to ensure moderate adhesion to the substrate andbetween adjacent printed layers. This process enables the 3D Au—Ag metalparticle structure 28 to be printed with virtually any 3D shape.

Referring to FIG. 4, after the 3D Au—Ag metal particle structure 28(i.e., green part) is printed using the DIW process, the structure maybe heated to anneal it, to form the alloy by interdiffusion of thedifferent metal particles, and burn off the organic binder 16. Dependingon the alloy melting point, the annealing temperature varies. Generally,anywhere from 0.99-0.7 of the melting temperature of the alloy to beformed may be used as the annealing temperature. The annealing time mayalso be varied as this variable depends on the particle size used in thealloy, and the annealing temperature. The annealing time may thus rangefrom 1 hour to 24 hours. Smaller particles and higher annealingtemperatures require a shorter time to form a homogenous alloy. In thisexample the annealing was performed by heating the structure 28 to 85020C. using a heating rate of 1020 C./minute, and annealed at thistemperature for twelve hours to remove the organic binder 16 and allowthe Ag and Au to form an alloy. The annealed structure 28 a′ is shown inFIG. 4 as well.

As indicated in FIG. 5, the annealed structure 28 a′ is then de-alloyed.The de-alloying process may be performed by any suitable process (e.g.,free-corrosion, electrochemical de-alloying, etc.). The de-alloyingprocess is often carried out in aqueous solution for free andelectrochemical de-alloying processes. Various types of acid andalkaline solution with the concentration from 1% to its saturated formcan be used for a free corrosion process. For an electrochemicalde-alloying process, besides the acid and alkaline solution, a neutralsolution such as NaCl, KCl, etc., with a concentration from 0.1 M to itssaturated form may be selected for electrochemical de-alloying processcontrolled by a potentiostat or power source (battery) with two or threeelectrode setups. For certain elements to form nanoporous structuressuch as Si, Ti, V, Cr, Fe, Co, Ga, Sn, Ta, Pb, and Bi, a meltde-alloying process may be used along with choosing a metallic elementwhich does not mix with the target element.

The melt de-alloying process starts with the target alloy by putting itinto a melting metal for certain time, and then taking it outside. Next,the treated piece may be exposed to an etching solution to remove theunwanted elements. The de-alloying in this example was performed bysubmerging the annealed structure 28 a′ in concentrated HNO3 solutionfor two days. In this example the process described herein resulted in ahierarchical metal foam morphology, represented by illustration 28 b′,with three distinct levels of pores (i.e., three distinct sectionshaving differing porosities). FIG. 6 shows a simplified cross-sectionalillustration of a portion of the 3D structure 28 b′ of FIG. 5 toillustrate the hierarchical pore architecture of the structure. Portions30 of the 3D structure 28 b′ may form macropores that operate asengineered mass transport “highways” or paths, while portions 32 formnanopores that provide the increased interior surface area that isexposed to reactants (ions or neutral species), and thus helps toprovide high (electro)catalytic reactivity.

The system and method disclosed herein may be used to fabricate a 3Dstructure having multiple levels of porosity, and in one specificexample three levels of porosity with a total porosity of 95% and asurface area of 5 m²/g, as shown in FIG. 7. FIG. 7 illustrates a portionof a 3D structure formed using the system and method of the presentdisclosure having a distinct first level macroscale porosity 34, adistinct second level mesoscale porosity 36 and a distinct third levelnanoscale porosity 38, while having a total porosity of 95% and asurface area of 5 m²/g. If desired, the engineered macroscale porositycan be made anisotropic to direct mass transport in applications thatrequire directional mass transport (for example flow batteryelectrodes). The system and method of the present application may beespecially useful in enabling manufacture of complex 3D structures thatwould be difficult and/or impossible to create using previouslyavailable manufacturing techniques. Further examples are 3D structures40-46 made using the teachings of the present disclosure as shown inFIGS. 8-11 respectively.

FIG. 12 shows a graph illustrating that the electrochemically accessiblesurface area of a 3D structure created using the teachings of thepresent disclosure is similar to that of a conventional unimodal 3Dstructure. FIG. 13 shows a graph illustrating faster charging responseas a consequence of a sudden jump of the applied electrochemicalpotential of 3D structures created using the teachings of the presentdisclosure, as compared to that of a conventional unimodal 3D structure.

Referring to FIG. 14, the above described operations performed using theteachings of the present disclosure are summarized in flowchart 100. Atoperation 102, if an ink is to be formed (i.e., rather than using apre-prepared feedstock), then the ink may be formed by mixing selectedquantities of two or more powdered metals together with an organicbinder. At operation 104 an additive manufacturing operation is thenperformed to create a 3D metal particle structure. At operation 106 anannealing operation is performed to burn off the organic binder and toform the alloy 3D structure. At operation 108 a de-alloying operation isthen performed on the annealed 3D structure which creates a 3D structurehaving a hierarchical metal foam architecture with several distinct poresizes.

The present invention thus uses DIW additive manufacturing to fabricatehierarchical nanoporous metal foams with deterministically controlled 3Dmultiscale porosities. Arbitrary yet mechanically robust 2D or 3D shapescan be printed according to the specific needs of the application.Moreover, the printed structure with its two, three or more distinctlevels of porosity can be tuned independently, in part by using the DIWoperation, in part by controlling the ink's organic binder content, inpart by controlling annealing of the structure and in part bycontrolling de-alloying of the structure, to create differentarchitectures for different layers or sections of the 3D structure,which enables application specific multiscale architectures to becreated. The ability of the present disclosure to create 3D metal foamswith deterministic shapes and a macroscale porosity is expected to havesignificant impact in the fields of energy storage for batteries,catalysis, and more. The methods disclosed herein can be used to createstructures such as filaments, films, and virtually any other type ofthree dimensional, monolithic or spanning free-form structures, where itis desired to have both high surface area and high electricalconductivity, in addition to two or more distinct pore size lengthscales.

It will also be appreciated that while the present disclosure hasdescribed a DIW process as being one example of the specific processbeing used to apply the ink 18, other fabrication processes in additionto DIW may be used as well. For example, the ink 18 may be used in moretraditional extrusion-based processes where the architecture is notcontrolled by the motion of the nozzles with respect to the XYZ stage,but by the shape of the nozzle itself. Furthermore, the presentdisclosure is not limited to use with only a DIW process; virtually anyform of additive manufacturing/3D printing method/process, for exampleand without limitation, Selective Laser Sintering, Selective LaserMelting, Binder Powder Bed Printing, Fused Deposition Modeling,Projection Microstereolithography, Electrophoretic Deposition, ScreenPrinting, Inkjet Printing, and other laser melting, sintering, ordeposition processes may be used in place of a DIW process. Virtuallyany process capable of producing multi-metal component parts with adigitally controlled macropore architecture, which may then be annealedto form the alloy, and then de-alloyed to create the functionalnanoporosity, is contemplated by the present disclosure.

It will also be appreciated that nanoporous metals can be prepared fromtypical binary and ternary alloys, or even from multi-composition alloys(i.e., more than three different elements). The less noble elements havea lower standard electrode potential compared with the more nobleelements for aqueous de-alloying process. Typical elements that can beused as less noble components are the following: Li, Na, Mg, Al, Si, K,Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Sr, Zr, Nb, Mo, Cd,In, Sn, Pb, Bi and most or non-radioactive rare earth elements. Typicalelements for the more noble elements to form nanoporous metals are: Si,Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Ga, Ge, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Sn,Ta, W, Os, Ir, Pt, Au, Pb, and Bi. Other elements such as Be, B, P, S,As, and Se can be used as additive elements. The typical elementcompositional range for the less noble element of the alloy is from 5 to99 atomic percent and the rest are the more noble elements. If the alloyparticles are available, then it would be possible to prepare thehierarchical nanoporous metals directly by using the alloy powders andbinders to form the macroscopic architecture.

While various embodiments have been described, those skilled in the artwill recognize modifications or variations which might be made withoutdeparting from the present disclosure. The examples illustrate thevarious embodiments and are not intended to limit the presentdisclosure. Therefore, the description and claims should be interpretedliberally with only such limitation as is necessary in view of thepertinent prior art.

Example embodiments are provided so that this disclosure will bethorough, and will fully convey the scope to those who are skilled inthe art. Numerous specific details are set forth such as examples ofspecific components, devices, and methods, to provide a thoroughunderstanding of embodiments of the present disclosure. It will beapparent to those skilled in the art that specific details need not beemployed, that example embodiments may be embodied in many differentforms and that neither should be construed to limit the scope of thedisclosure. In some example embodiments, well-known processes,well-known device structures, and well-known technologies are notdescribed in detail.

The terminology used herein is for the purpose of describing particularexample embodiments only and is not intended to be limiting. As usedherein, the singular forms “a,” “an,” and “the” may be intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. The terms “comprises,” “comprising,” “including,” and“having,” are inclusive and therefore specify the presence of statedfeatures, integers, steps, operations, elements, and/or components, butdo not preclude the presence or addition of one or more other features,integers, steps, operations, elements, components, and/or groupsthereof. The method steps, processes, and operations described hereinare not to be construed as necessarily requiring their performance inthe particular order discussed or illustrated, unless specificallyidentified as an order of performance. It is also to be understood thatadditional or alternative steps may be employed.

When an element or layer is referred to as being “on,” “engaged to,”“connected to,” or “coupled to” another element or layer, it may bedirectly on, engaged, connected or coupled to the other element orlayer, or intervening elements or layers may be present. In contrast,when an element is referred to as being “directly on,” “directly engagedto,” “directly connected to,” or “directly coupled to” another elementor layer, there may be no intervening elements or layers present. Otherwords used to describe the relationship between elements should beinterpreted in a like fashion (e.g., “between” versus “directlybetween,” “adjacent” versus “directly adjacent,” etc.). As used herein,the term “and/or” includes any and all combinations of one or more ofthe associated listed items.

Although the terms first, second, third, etc. may be used herein todescribe various elements, components, regions, layers and/or sections,these elements, components, regions, layers and/or sections should notbe limited by these terms. These terms may be only used to distinguishone element, component, region, layer or section from another region,layer or section. Terms such as “first,” “second,” and other numericalterms when used herein do not imply a sequence or order unless clearlyindicated by the context. Thus, a first element, component, region,layer or section discussed below could be termed a second element,component, region, layer or section without departing from the teachingsof the example embodiments.

Spatially relative terms, such as “inner,” “outer,” “beneath,” “below,”“lower,” “above,” “upper,” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. Spatiallyrelative terms may be intended to encompass different orientations ofthe device in use or operation in addition to the orientation depictedin the figures. For example, if the device in the figures is turnedover, elements described as “below” or “beneath” other elements orfeatures would then be oriented “above” the other elements or features.Thus, the example term “below” can encompass both an orientation ofabove and below. The device may be otherwise oriented (rotated 90degrees or at other orientations) and the spatially relative descriptorsused herein interpreted accordingly.

1. A system for using a feedstock to form a three dimensional,hierarchical, porous metal structure with deterministically controlled3D multiscale porous architectures, the system comprising: an reservoirfor holding the feedstock, the feedstock being formed as a rheologicallytuned alloy ink; a printing stage for receiving the feedstock; aprocessor including a memory and configured to help carry out anadditive manufacturing printing process to produce a three dimensional(3D) structure using the feedstock in a layer-by-layer fashion, on theprinting stage; a nozzle for applying the feedstock therethrough ontothe printing stage; a de-alloying subsystem for further processing the3D structure through a de-alloying operation to form a de-alloyed 3Dstructure having several distinct, differing pore length scales rangingfrom a digitally controlled macroporous architecture to a nanoporosityintroduced by the dealloying operation.
 2. The system of claim 1,wherein the feedstock comprises an alloy powder.
 3. The system of claim1, wherein the rheologically tuned alloy ink comprises an ink formedfrom a plurality of different metal powders and a binder.
 4. The systemof claim 1, wherein the additive manufacturing printing processcomprises a direct ink writing (DIW) process.
 5. The system of claim 1,wherein the additive manufacturing printing process comprises at leastone of: a direct ink writing (DIW) process; a selective laser sinteringprocess; a selective laser melting process; a binder powder bed printingprocess; a fused deposition modeling process; a projectionmicrostereolithography process; an electrophoretic deposition process; ascreen printing process; and an inkjet printing process.
 6. The systemof claim 3, wherein the rheologically tuned alloy ink comprises an inkformed from silver powder and gold powder.
 7. The system of claim 6,wherein the rheologically tuned alloy ink comprises also comprises anorganic binder.
 8. The system of claim 1, further comprising anannealing subsystem for performing an annealing operation on the 3Dstructure prior to performing the de-alloying operation.
 9. The systemof claim 8, wherein the annealing subsystem is configured to heat the 3Dstructure to 0.99%-0.7% of a melting temperature of an alloy being usedto form the 3D structure.
 10. The system of claim 9, wherein theannealing subsystem is configured to maintain the 3D structure heatedfor between 1 hour to 24 hours.
 11. The system of claim 1, wherein thede-alloying subsystem enables submerging the 3D structure in an aqueoussolution for a predetermined time.
 12. A system for forming a threedimensional, hierarchical, porous metal structure with deterministicallycontrolled 3D multiscale hierarchical pore architectures, the systemcomprising: a printing stage; an additive manufacturing system includinga processor having a nozzle, and configured to print a three dimensional(3D) structure in a layer-by-layer process by flowing a rheologicallytuned ink through the nozzle onto the printing stage and to build up the3D structure in a layer-by-layer; an annealing subsystem configured toanneal the 3D structure to remove the binder, and to form an alloyed 3Dstructure; and a de-alloying subsystem configured to de-alloy thealloyed 3D structure to form a hierarchical, nanoporous 3D structurehaving an engineered, digitally controlled macropore morphology withintegrated nanoporosity.
 13. The system of claim 12, wherein therheologically tuned ink comprises an ink from a plurality of metalpowders and a binder.
 14. The system of claim 12, wherein the annealingsubsystem is configured to heat the 3D structure to 99%-0.7% of themelting temperature of an alloy to be formed as the alloyed 3Dstructure.
 15. The system of claim 14, wherein the annealing subsystemis further configured to heat the 3D structure for a predetermined timeperiod from between 1 hour to 24 hours.
 16. The system of 12, whereinthe de-alloying subsystem is configured to enabling submerging thealloyed 3D structure in a solution.
 17. A system for forming a threedimensional, hierarchical, porous metal structure with deterministicallycontrolled 3D multiscale hierarchical pore architectures, the systemcomprising: a printing stage; a rheologically tuned, flowable inkincluding a metal powder and a binder; an additive manufacturing systemincluding a processor for controlling a printing process, and alsohaving a nozzle, and configured to print a three dimensional (3D)structure in a layer-by-layer process by flowing the rheologically tunedink through the nozzle onto the printing stage, to build up the 3Dstructure in a layer-by-layer printing operation; an annealing subsystemconfigured to anneal the 3D structure by heating the 3D structure for apredetermined time period to remove the binder, to form an alloyed 3Dstructure; and a de-alloying subsystem configured to de-alloy thealloyed 3D structure to form a hierarchical, nanoporous 3D structurehaving an engineered, digitally controlled macropore morphology withintegrated nanoporosity.
 18. The system of claim 17, wherein the metalpowder of the rheologically tuned ink comprises a mixture of a pluralityof different metal powders and the binder.
 19. The system of claim 17,wherein the annealing subsystem is configured to heat the 3D structureto 99%-0.7% of the melting temperature of an alloy to be formed as thealloyed 3D structure, for a predetermined period of time.
 20. The systemof claim 19, wherein the de-alloying subsystem is configured to enablingsubmerging the alloyed 3D structure in an aqueous solution for apredetermined period of time.